Without limiting the scope of the invention, its background is described in connection with the diffusion of dopants in the base layer of a heterojunction bipolar transistor (HBT), as an example.
Heretofore, in this field, it has been the object of device designers to develop HBTs to provide signal gain at microwave frequencies. A necessary consequence of this design goal is the formation of a structure that exhibits a low series base resistance, as base resistance is a major factor contributing to the maximum operating speed of the device. Generally, to lower the base resistance, designers have utilized a relatively heavy doping profile of the base layer. In base layers made from compound semiconductor materials, such as GaAs, high concentrations of p-type dopants such as Zn or Be are used.
It has been recognized that one of the factors that influence the reliability of microwave and high-speed heterojunction bipolar transistor (HBT) devices is the stability of the p-type dopant in the base layer. Heavy doping levels needed for good high frequency performance often cause a movement of commonly employed dopants under temperature and bias stress.
In the past, the use of carbon doping in metal organic chemical vapor deposition (MOCVD) systems has offered some relief to the diffusion problem since carbon has a low diffusivity in GaAs (a one-dimensional diffusion coefficient of 1-2.times.10.sup.-16 cm.sup.2 /s compared with 1.5-2.times.10.sup.-14 cm.sup.2 /s for Be and Zn). However, this solution presents inherent difficulties. For example, carbon doped GaAs has shown lattice constant change when incorporated at high levels (&gt;5.times.10.sup.18 cm.sup.-3) (see, for example, T. H. Chiu et al., "Chemical Beam Epitaxial Growth of Strained Carbon-Doped GaAs", Applied Physics Letters, vol. 57, p. 171, 1990). This lattice constant change can cause a degradation of device performance by lowering minority carrier lifetime or by increasing the interface state density due to stress built-in to the structure.
Accordingly, improvements which overcome any or all of these problems are presently desirable.